Sensor instrumental delay
Medtronic’s continuous glucose monitoring system uses data smoothening algorithm to suppress variation of ISIG values during calculation of interstitial fluid glucose values from sensor current according to formula {Principles 1}. This variation is caused by changes in interstitial fluid circulation at both sensor and cannula insertion sites. The algorithm takes into account several last ISIG values and its purpose is to smooth graphs shown on the pump display as shown on Fig. Delay 1.
Fig. Delay 1. Sensor glucose values smoothening
The figure shows example of ISIG values and their conversion to interstitial glucose values. If there could be no data smoothening, SG values calculated from ISIG would be in range from 7.5 to 8.0 mmol/l between 16:25 and 17:25. The data smoothening algorithm keeps them nearly constant at value of 7.8 mmol/l with exception of one value at 16:55.
On the other hand the data smoothening algorithm:
Fig. Delay 2. Delay between ISIG and interstitial glucose values
Fig. Delay 2. was created by recording of interstitial fluid glucose and ISIG values displayed by the pump. An afternoon snack was taken without bolus to create a hyperglycemic episode.
Fig. Delay 3. Display delay
The Fig. Delay 3. shows that the same values of the display delay repeat every 40 minutes. The display delay was 21 seconds at 18:45, 19:25, 20:05 and 20:45 or 42 seconds at 19:05, 19:45 and 20:25 and so on. The red line shows the pattern that values of display delay follow. It reaches 2 maximums and two minimums during the 40-minutes cycle. The difference between the minimums and maximums is around 98 seconds.
The display delay of 107 seconds at 19:00 does not fit the pattern. The additional delay was caused by the lost connection between the pump and the transmitter. The display delay value should be approximately 20 seconds lower according to the pattern.
I changed the pump time at 00:58:30 back to 00:57:00 to see how change of the pump time affects the display delay. The result is shown on Fig. Delay 4.
Fig. Delay 4. Display delay after time change
Comparison of Fig. Delay 3. with Fig. Delay 4. shows that the pattern is exactly the same and values of display delay are shifted by 30 seconds.
Analysis shows that display delay is composed of static and dynamic part. Static part can reach 0 – 59 seconds and it is caused by cutting of seconds from the times of mesurements. It can be reduced or enlarged by change of the pump time. Dynamic part can reach 0 – 98 seconds approximately. It is connected to the pattern shown on Fig. Delay 3. and Fig. Delay 4. and I do not have any other explanation that it is caused by error in pump software.
In the worst case, the display delay can reach 59 + 98 = 157 seconds.
The total shift between the glucose profile in capillary blood (measured by meter) and the interstitial fluid (measured by sensor and displayed by pump) is approximately 18 - 20 minutes in my case. The total shift is composed of: